1. Field of Invention
The present invention relates to pipeline plugs and in particular to apparatus or plug useful in forming a solid phase or ice plug in a pipeline for maintenance, pressure testing or the like purposes where a section of a pipeline is to be isolated from the rest of the pipeline.
2. Prior Art
The technique of forming solid phase plugs for isolation purposes of pipelines using liquid nitrogen as cooling media is described in U.S. Pat. No. 4,220,012 issued Sep. 2, 1980. It employs a chamber placed over the line pipe surface. The chamber is then filled with liquid nitrogen. The charge is agitated to induce boiling thus increasing heat flow from the line pipe to the evaporating nitrogen. This method uses pipeline fluid or water to form a solid phase plug. The time to form solid phase plug using this method is long because the ice which forms the plug occupies the full section of the line pipe. The apparatus of the present invention is also referred to in the art of pipeline maintenance or testing as an "icepig" or an "ice plug".